Tumors characteristically express atypical, potentially immunoreactive antigens that are collectively referred to as tumor antigens. Accumulating evidence suggests that the failure of the immune system to mount an effective response against progressively growing tumors is not attributable to a lack of recognizable tumor antigens. Immunosuppression by tumors is poorly understood and mechanisms by which tumors may escape immune surveillance have been poorly explored. Recently, it has been shown that cytotoxic T cells become tolerized by a reduction in local concentrations of tryptophan that are elicited by indoleamine 2,3-dioxygenase (IDO) activity.
IDO is an oxidoreductase that catalyzes the rate-limiting step in tryptophan catabolism. This enzyme is structurally distinct from tryptophan dioxygenase (TDO), which is responsible for dietary tryptophan catabolism in the liver. IDO is an IFN-γ target gene that has been suggested to play a role in immunomodulation (Mellor and Munn (1999) Immunol. Today, 20:469-473). Elevation of IDO activity depletes the levels of tryptophan in local cellular environments. Induction of IDO in antigen-presenting cells, where IDO is regulated by IFN-γ, blocks the activation of T cells, which are especially sensitive to tryptophan depletion. T cells must undergo 1-2 rounds of cell division to become activated, but in response to tryptophan depletion they arrest in G1 instead. In this way, IDO has been proposed to inhibit the TH1 responses that promote cytotoxic T cell development.
The main evidence for the role of IDO in immunosuppression is demonstrated by the ability of 1-methyl-tryptophan (1MT), a specific and bioactive IDO inhibitor (Cady and Sono (1991) Arch. Biochem. Biophys. 291:326-333), to elicit MHC-restricted and T cell-mediated rejection of allogeneic mouse concepti (Mellor et al. (2001) Nat. Immunol. 2:64-68; Munn et al. (1998) Science. 281: 1191-93). This effect is consistent with the high levels of IDO expression in placental trophoblast cells (Sedlmayr et al. (2002) Mol. Hum. Reprod. 8:385-391).
Significantly, IDO activity has been shown to be elevated frequently in human tumors and/or in cancer patients (Yasui et al. (1986) Proc. Natl. Acad. Sci. USA. 83:6622-26; Taylor and Feng (1991) FASEB J. 5:2516-22). Since IDO can modulate immune responses, one logical implication is that IDO elevation in cancer may promote tumor immunosuppression (Mellor and Munn (1999) Immunol. Today, 20:469-473; Munn et al. (1999) J. Exp. Med. 189:1363-72; Munn et al. (1998) Science. 281:1191-93). This possibility is supported by the observation that many cancers, including breast cancer, are characterized by a loss of beneficial immune functions that can limit malignant development. For example, TH1 responses (of which IFN-γ production is a hallmark) that promote the production of cytotoxic T cells are suppressed during cancer progression. A resultant hypothesis from this data was that if IDO drives cancer progression by blunting T cell activation, then IDO inhibition in animals should blunt tumor growth by reversing IDO-mediated immunosuppression. However, delivery of the IDO inhibitor 1-methyl-tryptophan (1MT) only retarded and did not prevent tumor growth in a mouse model (Friberg et al. (2002) Int. J. Cancer 101:151-155; U.S. Pat. No. 6,482,416).
Cellular signal transduction, i.e., the series of events leading from extracellular events to intracellular sequelae, is an aspect of cellular function in both normal and disease states. Numerous proteins that function as signal transducing molecules have been identified, including receptor and non-receptor tyrosine kinases, phosphatases and other molecules with enzymatic or regulatory activities. These molecules generally demonstrate the capacity to associate specifically with other proteins to form a signaling complex that can alter cellular proliferation.
Aberrant signal transduction can lead to malignant transformation, growth, and progression. Accordingly, inhibitors of signal transduction pathways have been used to treat cancer. During the past few years, a number of signal transduction inhibitors (STIs) have been developed and their ability to suppress tumor growth is currently under investigation.